Semi-conductor switching devices are becoming available at lower costs and with higher power level switching capacities. As a result, the semi-conductor switches find ever increasing use for controlling commutation in brushless DC motors and variable freqency AC motors. These motors can be constructed without moving contacts such as brushes, slip rings, or commutators. Such motors have the advantage of high operating speeds and increased useful life. Furthermore, such motors are more readily utilizable with electronic control systems.
In order to achieve the benefits of the solid state commutation circuitry when the motors are used in position or velocity servo systems, however, it is necessary to also utilize a compatible rate feedback device which likewise avoids mechanical commutation or other moving mechanical contact switches.
Brushless motors are capable of reaching higher speeds than conventional DC motors The same is true of brushless rate sensitive feedback devices. A conventional DC tachometer however cannot reach similarly high speeds due to brush bounce at high speeds caused by small commutator eccentricities.
Brushless motors and tachometers have a longer time between failures since the only moving parts are the rotory shaft bearings. In a conventional DC motor or tachometer the operating life between repair is normally determined and limited by the contact wear of the moving contacts.
Brushless motors can operate in adverse environments (explosive or corrosive environments in particular) since there are no contacts which can cause arcs. This advantage of the motor is lost, however, when a conventional DC tachometer with moving contacts is used in the system.
Conventional tachometers also have a volts per commutator bar limitation which limits the output potential over a given speed range. As a result, such tachometers cannot achieve the higher sensitivities potentially available with brushless rate sensing devices.
In the past, several methods of achieving brushless rate feedback for position and velocity servos have been devised.
One such method employs an incremental sensor which indicates each increment of movement and produces a signal having a frequency proportional to speed. The frequency signal is then converted to either an analog signal proportional to speed or a digital word indicating speed. Such systems are relatively inexpensive but lack precise speed information at very low speeds near zero RPM. In order to provide any rate information at very low speeds (other than zero RPM) the rotor must move until a mark is detected. The time between mark detection can be too long to provide a meaningful frequency indication of speed. Also, such systems do not inherently provide directional information and become more complex when directional information is required.
Another approach is to use a resolver to indicate shaft position. Resolvers provide AC wave forms according to the sine and cosine of the shaft position. These wave forms can be converted into digital words or analog signals proportional to shaft position which in turn can be used to provide rate information. This method provides relatively good results but is quite expensive, particularly if low speed accuracy is required.
Optical encoder systems have also been employed. In such systems a digital word appears at the encoder output representing the shaft position and this information can be processed to provide rate information. This approach is less expensive than that using resolvers, but suffers from low speed limitations similar to those with the incremental sensor mentioned above.
Still another approach is to use an alternator with rotating permanent magnets. The signal produced by the stator windings is proportional in amplitude and frequency to the shaft speed. Where directional speed information is not required, diodes rectifying the alternator output will provide a DC signal proportional to speed. Bi-polar switching transistors controlled by position sensors can be used to get a bi-directional speed indication. However, these approaches using an alternator have a dead band near zero RPM because of the threshold conduction properties of diodes and transistors used in such systems.